Scale formation in the apparatus employed in the making of kraft papers is especially prevalent under the conditions ordinarily employed, namely the cooking of the wood chips at high temperatures in aqueous media containing caustic, sodium carbonate, and sodium sulfide. Under such conditions the calcium present in the water can react with the carbonate and produce rapid scaling from deposition of CaCO.sub.3 which can constrict the slots of the digester screens and cause unscheduled shutdowns of the equipment.
In addition to the calcium in the water, scale forming amounts of calcium, manganese, sulfur and magnesium may be leached from the wood chips as they pass through the digester. Organic constituents, found naturally occurring in the wood, are also leached in the digester and some of these organic constituents apparently have a chelating effect and actually prevent the precipitation of mineral scale e.g. CaCO.sub.3, at low temperatures. The metal-chelant complexes so-formed probably decompose at the higher temperatures during the cooking process, permitting the precipitation of inorganic scales. Some organic constituents of the wood also produce scale which deposits in polymeric form, and is commonly referred to as "pitch". Pitch deposit is often a problem, but this occurs most often at the lower temperatures and pH ranges encountered after the cooking step in the digester in a kraft mill.
Various additives have been employed in efforts to control the deposition of scale. Low molecular weight polyectrolytes have been employed. In particular, the use of a proprietary mixture of low molecular weight anionic polyelectrolytes and sequestrants is reported in an article in Canadian Pulp and Paper Industry, April, 1976, pp. 25-27. The article reports an improvement in cleaning frequency of from weekly before treatment to one to two months after treatment. The length of time required to clean the digester heater was also reduced.
Phosphonates which have been commonly used to control scale include nitrilotri(methylenephosphonic acid) (NTMP), diethylenetriaminepenta(methylenephosphonic) acid (DTPMP) and 1-hydroxyethane-1,1-disphosphonic acid (HEDP). These compounds are used in the acid form or the form of their water soluble salts, e.g. ammonium and alkali metal salts. In U.S. Pat. No. 4,253,912 there is disclosed the use of certain oligomeric phosphonic acid derivaties. These are condensate oligomers prepared by reacting an alkanoyl chloride or an organic acid anhydride with a 1-hydroxyalkane-1,1-disphosphonic acid compound. In an example, phosphorous acid and propionic anhydride are reacted to produce one such oligomer; another is formed by reacting with heating phosphorous acid and decanoyl chloride and subsequently pouring the thick melt formed into acetic anhydride. These oligomeric products are said to inhibit the precipitation of calcium at threshold levels.
Other additives have been employed in aqueous systems to control deposition of scale. Thus, for example, in cooling water systems and in pulp and paper mill applications phosphonate-type chelants have been used.
In U.S. Pat. No. 4,406,811 there is disclosed a composition for control of corrosion and scale in aqueous systems containing multiple metallurgies, e.g. industrial cooling equipment. Such compositions contain (a) a triazole or a mercaptotriazole, (b) an aliphatic mono- or dicarboxylic acid, (c) a nonionic wetting agent and (d) a phosphrous-containing organic scale inhibiting compound, eg. HEDP or NTMP.
It has now been discovered that certain polyaminopoly(methylenephosphonic acid) compounds together with certain nonionic surfactants provide excellent control of scale due to calcium under the extreme temperature conditions found in pulp digesters.